Most extensively used in the art are metal - oxide anodes with an active coating containing 30 mol. % of ruthenium dioxide and 70 mol. % of titanium dioxide known as DSA (dimensionally stable anodes), in the USSR they are referred to as ORTA (a trademark registered in the Soviet Union). In the USSR ORTA electrodes are protected by USSR Inventor's Certificate No. 369923.
The consumption of the active mass of this electrode under stationary conditions of chlorine electrolysis at a current density of 0.2-0.4 A/cm.sup.2, as determined by the radiochemical method, is equal to 2.6.times.10.sup.-8 g/cm.sup.2.h.
The resistance of the active coating of such an electrode can be determined by the method of variable polarity and amalgamation which is widely employed as a fast method for evaluation of quality of an active coating: resistance to amalgamation, adhesion to the current-conducting substrate, resistance to cathodic polarization and short-circuit resistance.
The results of measurements of the mass consumption of an active coating of an ORTA electrode obtained by the method of variable polarity and amalgamation are shown in the Table hereinbelow.
An ORTA electrode, as compared to a graphite electrode, under the conditions of electrolysis of alkali metal chlorides makes it possible to lower an overvoltage across an electrolyzer and save about 200 kW.h per ton of caustic soda (as calculated for a 100% product), to improve the purity of the electrolysis products, to extend the service life of an anode from 7-8 months to 5-7 years, well as to reduce the expenses for disassembling and reassembling of an electrolyzer for replacement of the electrodes.
TABLE ______________________________________ Number of test cycles 1-3 4-6 7-9 10-12 13-15 16-18 ______________________________________ Consumption of 0.595 0.610 0.140 0.180 0.190 0.170 the active mass for every 3 suc- cessive cycles of tests, mg/cm.sup.2 ______________________________________
However, ORTA electrodes have the following disadvantages: a relatively high consumption of noble metals especially noticeable in a mass operation of these electrodes; an insufficient resistance of the coating under the conditions of combined evolution of oxygen and chlorine; at an elevated content of oxygen in the anode gas "closing" of an electrode occurs at still relatively high residual content of ruthenium in the active coating of the electrode. These factors lower the electrode operation reliability, especially under the conditions of electrolysis with an ion-exchange membrane.
Known in the art is an electrode for electrochemical processes comprising an electrically conducting substrate of titanium or tantalum, onto which an active coating is deposited consisting of a platinum group metal oxide and a mixture of oxides of metals containing an oxide of titanium or tantalum and at least one more oxide of an alloying metal selected from the group consisting of: tin, silver, chromium, lanthanum, aluminium, cobalt, antimony, molybdenum, nickel, iron, tungsten, vanadium, phosphorus, boron, beryllium, sodium, calcium, strontium, lead, copper, and bismuth. The alloying metal oxide is used in an amount of 0.1 to 50% by weight of titanium dioxide or tantalum pentoxide. The ratio of the content of a platinum group metal to all other metals of the oxide coating is equal to 20:100-85:100. In the case where the active mass of the a prior art electrode contains TiO.sub.2, RuO.sub.2 and SnO.sub.2 the proportions of these components in molar per cent will be as follows: 40-90% of TiO.sub.2, 0.25-25% of SnO.sub.2 and 9.75-35% of RuO.sub.2. A known electrode of the following composition, molar per cent: 21.8% of RuO.sub.2, 72.7% of TiO.sub.2 and 5.5% of SnO.sub.2 after 1,500 hours of tests in a concentrated solution of NaCl at 2 A/cm.sup.2 and temperature of 60.degree. C. has an anode potential of 1.42 V. Under test conditions of the variable polarity method (5 anodic and 5 cathodic polarizations at 1 A/cm.sup.2, 2 minutes for each polarization) this electrode has a weight loss of 0.09 mg/cm.sup.2 after two test cycles and weight loss of 0.01 mg/cm.sup.2 after one immersion into amalgam (cf. U.S. Pat. No. 3,948,751 Cl.C 25 B 11/10, 1976).
This electrode has a disadvantage residing in its low resistance.
Also known in the art is an electrode for the electrochemical production of chlorine and an alkali comprising a substrate of a rectifying metal with an active coating deposited thereon and consisting of a mixture of oxides of tin, ruthenium and titanium, the molar ratio of TiO.sub.2 : :(RuO.sub.2 +SnO.sub.2) being within the range of 1.5-2.5:1 and the content of SnO.sub.2 being equal to 35-50 mol. % in a mixture of SnO.sub.2 +RuO.sub.2 (cf. U.S. Pat. No. 3,855,092 Cl.204-128, 1975).
This electrode has the following composition of the active coating, molar per cent: TiO.sub.2 --60-75%, SnO.sub.2 --10-20%, RuO.sub.2 --15-30%.
A known electrode with the active coating of the following composition: TiO.sub.2 : (RuO.sub.2 +SnO.sub.2)=2.2:1 (in molar parts) and at the content of SnO.sub.2 of 40 mol. % in a mixture of RuO.sub.2 +SnO.sub.2, i.e. the active mass having the composition: TiO.sub.2 67%, (molar per cent), SnO.sub.2 --13.2%, RuO.sub.2 --19.8%, has under the conditions of chlorine electrolysis the loss of ruthenium from the active mass equal to 0.01 g per ton of chlorine. Overtension of chlorine evolution on this prior art electrode is lowered by 40 mV relative to the electrode containing no SnO.sub.2 in the active coating.
This prior art electrode also has a disadvantage residing in its insufficient resistance.